When it comes to searching, I try to keep in mind what Tim Miller always says: Each area has to be searched at least 3 times or more because things are missed for a variety of reasons. I don't know what happened with the canal, but several posters have offered ideas on how this young boy may have escaped detection.
As for the little girl in OK, it appears she was found outside the search perimeter. That indicates they did not make the search area large enough at the beginning.... this must be a really tough call. She was two... how far could she go on her own? But anyway, it sounds as if they are going to review procedures to see how/why the area was not included.
Yes, I do think everybody should take responsibility for doing their job and doing it well - but in some cases, doing it well may not be as easy as it looks.
I do hope whatever OK finds when they review their procedures - they share it with the rest of the country and organizations like Equusearch, Klaas Kids, etc. in an effort to prevent such things from happening again.
Salem